Faraday rotator

ABSTRACT

A Faraday rotator for a Faraday isolator with an input polarizer, with an output polarizer, with a roller-shaped optical crystal that is arranged therebetween and that is arranged symmetrical to its axis of symmetry, with a right hollow cylinder that surrounds this and has a hollow space made of a permanent magnetic material, which cylinder is axially magnetized and the magnetic field of which extends in the hollow space approximately parallel to the axis of symmetry that runs in only one direction from the north pole to the south pole, and with terminal magnets attached to each of the two end faces in the plane perpendicular to the y- and z-directions of the axis of symmetry, each of which is embodied as a hollow right cylinder and has a through-aperture in the extension of the axis of symmetry, is characterized in that each terminal magnet is largely radially magnetized with regard to the axis of symmetry at least by region, in that the one of the two terminal magnets is magnetized radially from interior to exterior and the other terminal magnet is magnetized radially from exterior to interior, and in that the hollow cylinder at its north pole is adjacent to the terminal magnet that is magnetized from interior to exterior and at its south pole is adjacent to the terminal magnet that is magnetized from exterior to interior.

BACKGROUND OF THE INVENTION

The invention relates to a Faraday rotator for a Faraday isolator,namely such a Faraday isolator with an input polarizer, with an outputpolarizer, with a roller-shaped optical crystal that is arrangedtherebetween symmetrical to its axis of symmetry, with a right hollowcylinder that surrounds this and is made of a permanent magneticmaterial that is axially magnetized and the magnetic field of whichextends in the hollow space approximately parallel to the axis ofsymmetry that runs in only one direction from the north pole to thesouth pole, and with terminal magnets, attached to each of the two endfaces in the plane perpendicular to the y- and z-directions of the axisof symmetry, that are embodied as hollow vertical cylinders and have athrough-aperture in the extension of the axis of symmetry.

Faraday isolators, also called optical isolators, have the object ofpermitting a laser beam to pass in only one direction. For this, it hasan optical rotator, also called a Faraday rotator, polarizers beingmounted on both the input and output thereof, and their direction ofpolarization to one another forms a 45° angle. In general the Faradayrotator comprises a roller-shaped crystal made of a magnetoopticalmaterial (for instance TGG). The crystal is surrounded by a hollow rightcylinder made of a permanent magnetic material that generates a magneticfield that runs along the axis of symmetry of the crystal. The Faradayeffect occurs in that the direction of polarization of the incominglaser beam is rotated by a certain angle when it passes through thecrystal. The direction of rotation of the polarization direction isindependent of the propagation direction of the laser beam. The size ofthe angle of rotation is a function of one of the characteristicconstants for the material of the optical crystal. This itself is afunction of the wavelength of the laser beam. The angle of rotation ofthe direction of polarization during operation is adjusted such that itis approximately 45°. The output polarizer is also arranged rotatedabout this angle, and in addition transmits the maximum radiationintensity. A beam that runs against the propagation direction passes theoutput polarizer and is rotated 45° (in the same direction), that is, atotal of 90°, by the Faraday rotator, so that high quenching, alsocalled extinction, is effected for the returning laser beam. In order toincrease this further to a higher extinction, so-called two- or evenmulti-stage Faraday isolators are used in which the extinction isfurther enhanced.

Such a generic Faraday isolator is known in and of itself. Theroller-shaped magnetooptical crystal is surrounded by a right hollowcylinder with a circular cross-section and made of permanent magneticmaterial that is polarized magnetically in the axial direction. Oneterminal magnet, in the form of a right hollow cylinder with a circularcross-section, can be connected on either side to the two end surfacesof this hollow cylinder, which are both magnetized parallel to the axisof symmetry of the magnetooptical crystal, that is, also in the axialdirection, like the hollow cylinder surrounding the crystal. Inaddition, the two terminal magnets are magnetized axially in the samedirection to one another and with reference to the hollow cylinderopposite the hollow cylinder as central magnet.

Such a generally known Faraday isolator has proved itself. However, amore compact structure is not possible in order to attain the necessarymagnetic field strengths in the magnetooptical crystal.

The object of the invention is therefore to embody more compactly ageneric Faraday isolator with good homogeneity of the magnetic fieldstrengths.

SUMMARY OF THE INVENTION

This object is inventively attained in a generic Faraday isolator inthat each terminal magnet is largely radially magnetized with regard tothe axis of symmetry at least by region, in that the one of the twoterminal magnets is magnetized radially from interior to exterior andthe other terminal magnet is magnetized radially from exterior tointerior, and in that the hollow cylinder at its north pole is adjacentto the terminal magnet that is magnetized from interior to exterior andat its south pole is adjacent to the terminal magnet that is magnetizedfrom exterior to interior.

Thus, in the inventive principle the center cylinder magnetized parallelto the axis of symmetry in the axial direction is essential. Itsmagnetic field strengths are amplified in the hollow space of thecylinder (that is, in the region of the crystal) by the two terminalmagnets in the region of the contact location to the—center—cylindersuch that a higher magnetic field strength results across the axiallength of the crystal.

This inventive principle has the advantage that substantially smallerstructures can be provided for the Faraday rotator, both in the axialand in the radial direction, so that a compact structure results overallfor the Faraday isolator in accordance with the invention. In order toincrease this further to a maximum extinction at which the still-presentlast transmission can be suppressed below negligible values, theinvention can also be used in two- or multi-stage Faraday isolators.

The two terminal magnets can either be fitted as one-piece right hollowcylinders with a circular cross-section and with a magnetic fieldideally directed radially with regard to the axis of symmetry, or cancomprise individual parts that are largely sector-shape incross-section, like wedges of pie, in which a uniform orientation of themagnetic field in one direction, parallel to the plane of symmetry(which passes through the axis of symmetry of the crystal) of the piewedge-shape part. Such a design would be attained if the pie wedge-shapepart is cut out of a rectangular permanent magnet with uniform magneticfield.

Using approximately radially magnetized magnets with Faraday isolatorsis known (U.S. Pat. No. 5,528,415). Apart from the fact that thesemagnets comprise four radiation-symmetrical parts that have atrapezoidal cross-section, while leaving free an aperture that is squarein cross-section, that is, in contrast to the invention, that do notsymmetrically include the roller-shape crystal, there is no centercylinder of the permanent magnets in this known embodiment, whichhowever is essential for the invention due to the overlaying effect atthe contact location. In addition, the two radially magnetizable magnetsare arranged spaced from one another in the axial direction so thatconsequently only a weak overlaying effect can occur in the vicinity ofthe two magnets. In one useful embodiment, the terminal magnets that areat least largely radially magnetized by region are magnetized such thatthey also possess a component in the direction of the axis of symmetryof the crystal. Because of this, there is a further enhancement of thestrength of the magnetic field in the hollow region of the cylindercompared to the generic prior art.

Additional useful embodiments and further developments of the inventionare characterized in the subordinate claims.

One preferred exemplary embodiment of the invention is explained in moredetail in the following with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective drawing of the Faraday rotator in accordancewith the invention;

FIG. 2 is a schematic head-on view of the Faraday rotator in accordancewith FIG. 1;

FIG. 3 is a view III—III in accordance with FIG. 2;

FIG. 4 is a section IV—IV in accordance with FIG. 3;

FIG. 5 is a section V—V in accordance with FIG. 4;

FIG. 6 is the detail VI in accordance with FIG. 2;

FIG. 7 a is the section VII—VII in accordance with FIG. 6, as a firstembodiment; and,

FIG. 7 b is the section VII—VII in accordance with FIG. 6, in a secondembodiment.

DETAILED DESCRIPTION OF THE INVENTION

The Faraday isolator in FIG. 1, labeled 10 as a whole, is constructedsymmetrically with respect to its axis of symmetry (x). It has aright—center—cylinder 11 that has a circular cross-section (FIG. 1 a),in the cylindrical hollow space 12 of which is arranged themagnetooptical crystal, labeled 13 as a whole (FIG. 1 b). The crystalcan extend in the axial direction across the entire axial length of thecylinder 11 to the two end faces 14, which are in the two planes thatextend perpendicular to the axis of symmetry x through the y and z axes.The cylinder 11 comprises a permanent magnetic material and is embodiedwith its magnetic field B parallel to the axis of symmetry x (FIG. 1 b).

As can be seen in FIG. 1 a, attached to the two end faces 14 are the twoterminal magnets 16 and 17, which are both formed like the cylinder 11as hollow, vertical cylinders that have a circular cross-section andhave a through-aperture 18 extending the axis of symmetry x (see alsoFIG. 2).

The hollow cylinder 11 as a permanent magnet is illustrated in greaterdetail in FIGS. 4 and 5. Clearly visible is the magnetic field strengthB, which is oriented parallel to the axis of symmetry x and the northpole N of which is located on the one end face 14 and the south pole Sof which is located on the opposing end face.

In the exemplary embodiment illustrated, neither of the two terminalmagnets 16, 17 is embodied in one piece and precisely radiallymagnetized, but rather each comprises eight pie wedge-shape parts 19 and20 that with respect to the axis of symmetry x are largely radiallymagnetized, and radiation-symmetrical. One such part 19 (FIG. 2) isillustrated in greater detail and in a larger scale in FIG. 6. The axisof symmetry x here is perpendicular to the drawing plane, which in thesection illustrates the part that is in the y/z plane and that up to thethrough-aperture 18 is largely in the shape of a sector.

Two different embodiments are illustrated in the section VII—VII inassociated FIGS. 7 a and 7 b. In the embodiment in accordance with FIG.7 a, the magnetic field is oriented without a component in the directionof the axis of symmetry x, that is, only in the y/z plane. Thisembodiment possesses a stronger magnetic field in the region of thecrystal 13 compared to the generic prior art.

If, in addition, the entire magnetic field B and the axis of symmetry xform an angle other than 90°, even better results occur than in theembodiment in accordance with FIG. 7 b, (which results however areattained using increased production complexity). Within one part 19 or20, the orientation of the magnetic field B is parallel and, oriented inthe sectional plane VII—VII in accordance with FIG. 6, that forms themirror symmetry plane for the part 19 of the terminal magnet 16illustrated in FIG. 6.

The arrangement and polarization of the terminal magnets to the axiallymagnetized cylinder 11 is essential.

As is illustrated in FIGS. 6 and 7, each part 19 and 20 of the terminalmagnets 16 or 17 is magnetized somewhat radially. In addition, the oneterminal magnet 17 is then magnetized from interior to exterior (thatis, with the south pole S in the outer-most partial cover region), whilethe other terminal magnet 16 is polarized from exterior to interior,that is, with the north pole in the exterior partial cover region, asdepicted schematically in FIGS. 2 and 3.

Finally, in accordance with the teaching of the invention, the hollowcylinder 11 must at its north pole N be adjacent to the terminal magnet17 magnetized from interior to exterior and at its south pole S to theterminal magnet 16 magnetized from exterior to interior, as illustratedin FIGS. 2 and 3. This is the only manner in which the inventive resultsare attained.

1. A Faraday rotator for a Faraday isolator, the Faraday rotatorcomprising a roller-shaped magnetooptical crystal having an axis ofsymmetry, a right circular hollow cylinder comprised of a permanentmagnetic material surrounding the crystal, the cylinder being axiallymagnetized whereby a magnetic field thereof approximately parallel tothe axis of symmetry extends into the hollow of the cylinder, themagnetic field running in only one direction from a north pole to asouth pole, and respective right cylindrical permanent magnets attachedto respective end faces of the hollow cylinder-surrounded crystal, eachof said end face magnets having an aperture therethrough which iscoaxial with the axis of symmetry, wherein at least a region of one ofthe end face magnets at the north-magnetized end of the axiallymagnetized cylinder is radially magnetized from interior to exteriorwhereby a magnetic field of said one end face magnet has its north poleradially inward and its south pole radially outward and at least aregion of the other end face magnet at the south-magnetized end of theaxially magnetized cylinder is radially magnetized from exterior tointerior whereby a magnetic field of said other end face magnet has itssouth pole radially inward and its north pole radially outward.
 2. AFaraday rotator according to claim 1, wherein said regions aresubstantially sectors.
 3. A Faraday rotator according to claim 1,wherein said regions are in the form of respective discrete parts which,when assembled with other parts, form the respective end face magnets.4. A Faraday rotator according to claim 3, wherein the discrete partsare each substantially in the shape of a respective sector.
 5. A Faradayrotator according to claim 1, wherein the respective magnetic fields ofthe end face magnets are oriented obliquely with respect to the axis ofsymmetry.